Namely, is it possible to have too much iron in the water for plants? I ask
because our raw well water has 18ppm (yes, 18, not .18). After it has been
let to stand for a while, it turns a real nice caramel color, heh. I am
trying to stay away from having to use R/O water and I have proven to myself
I can't reliably use the water from our filtration system (softened big
time). By default our water is soft, acidic, with a very high iron content
with some tannins. The soft/acidic part I can take care of easily as long
as the iron isn't going to be a killer for me.

Not sure if there can be too much for the plants, but I know it will be
toxic to the fish at some point, which I am sure will vary with the
species. I have a well here too that we use for outdoor watering, and your
water sounds similar (orange :-). I found that a lot of the iron in the
water was actually in the form of fine particulates and a lot of shale, and
I now separate out a *lot* of it using a "spin-down" type of filter. The
spin-down filter makes a vortex inside the pushes the particulates to the
outer perimeter of the filter and the outlet is in the center of the unit.
The result is that the particulates precipitate down to the sediment trap
in the bottom of the unit and cleanER water (this is not an RO system)
comes out. It has a screen inside but not really a conventional filter
cartridge and it is simple to clean. I got one for about $50-60 from a
place in Florida that sells them for irrigation use. I installed it to keep
little mister-type sprinklers from clogging but I've found it does clear
the water somewhat although I haven't done any "real" testing on the water
either coming in or going out of the unit.

You might want to try one of the spin-down filters and see if it helps
clear your water a bit. Even if it doesn't make a difference, it will
separate out a lot of the particulates that would cause an RO system to
clog more quickly.

Also, if the plants (hopefully fish too) are ok with that much iron, will
they suck it out of the water in time and get rid of the coloration?

I don't think the plants will take enough iron out of the water. My
understanding is that the well water iron is not directly useable by the
plants. You most like do not want to use softened water since the softener
is just replacing Ca and Mg ions with Na ions which doesn't make it any
better for a planted tank.

-Bill

In Detroit, MI where everything is white with slick ice on top, and a lot
of wind today too.